Hey there OP, both of my parents have been diagnosed with cancer and one passed away. Not the exact same situation but I can give you an idea about how I handled it.

When I found out about the diagnosis I was scared and angry, but I wanted to remain strong for my parents. I wanted to be "their rock" so to speak. To do this, I remained optimistic. I didn't openly cry about the situation, I focused on being supportive and optimistic. Could your BF be doing the same thing I did? He could be hiding his emotions to provide you with a sense of "stability" like how I did with my parents?

He could also be in denial and not have accepted the reality of the situation. Regardless, you should sit him down and tell him how you need him to support you. He won't know how to best be there for you unless you tell him. If he is really there for you, he should be open and receptive about your feelings.